The disclosed invention is directed generally to holograms for center-high mounted stop lights for vehicles, and more particularly to a hologram that efficiently meets the required brightness and angular coverage and to a double exposure technique for recording such hologram.
Present federal regulations require center-high mounted stoplights (CHMSLs) in addition to the standard stoplights mounted in the rear portion of an automobile. The high mounted stoplights are intended to maximize the visibility of the automobile braking indicators to drivers following the braking vehicle, and are commonly mounted on the rear window of an automobile.
High mounted stoplights have commonly been implemented as a standard lenticular lens and an illuminating incandescent bulb enclosed in a housing that is typically secured adjacent the top or bottom of an automobile rear window. However, the bulky housing partially obscures the rearward vision of the driver, and moreover imposes limitations on the design of the automobile.
Center high mounted stoplights have been also integrated into automobile body parts such as rear decks, spoilers, roofs, which to some degree substantially reduce or remove the rearward vision problem. However, such stoplights are complex and may impose limitations on the design of the automobile.
Holographic center-high mounted stoplights have also been developed in order to efficiently meet the stoplight regulations. The hologram for holographic center-high mounted stoplights is typically a diffusion hologram or an image hologram that contains the image of a lenticular lens.
A consideration with holograms for holographic center-high mounted stoplights are the luminous intensity and angular coverage requirements. Such requirements generally include a quantitative aspect that defines luminous intensity over a defined solid angular range, and a qualitative aspect that requires visibility over a horizontal angular range that is greater than the horizontal component of the defined solid angular range. Essentially, the regulations require a brighter region generally in the center of the angular region of coverage.
A straightforward procedure for recording a hologram for a center-high mounted stoplight would be to record the image of a lenticular lens array that spreads light uniformly over a selected angular coverage (e.g., two lenticular arrays rotated 90 degrees with respect to each other and laminated together). If a relatively narrow angular coverage is selected so as to meet the central region luminous intensity requirements, the visibility requirement over the extended horizontal angular range might not be met. If a lenticular lens array of wider angular coverage is utilized, a stronger playback source would have to be utilized to meet the central luminous intensity requirements since the illumination would be diffracted uniformly over the selected angular coverage that is greater that the central region.
A further approach would be to utilize a lenticular lens array having a cylindrical lens facets configured so that the refracted output light was concentrated mostly in the center and less in the region outside the center. A consideration with such arrays include complexity and cost.